Reinforcement in evolutionary biology describes a process where natural selection strengthens reproductive isolation between two diverging populations. This occurs when these partially separated groups come into contact and produce low-fitness offspring. Selection favors traits that reduce unproductive interbreeding, enhancing genetic separation between the populations.
Understanding Reinforcement in Evolution
Reinforcement operates when two diverging populations meet in a shared geographic area, often referred to as a hybrid zone. If individuals from these populations interbreed and produce hybrids that are less fit, natural selection acts against those pairings. This selective pressure favors individuals who avoid mating with the other population.
The underlying mechanism involves natural selection promoting the evolution of “pre-zygotic isolation” traits. Pre-zygotic isolation refers to differences in behavior or biology that prevent hybrid zygote formation. Individuals who possess traits that help them recognize and mate only with their own kind will have greater reproductive success, as they avoid wasting reproductive effort on producing unproductive offspring. Over time, these traits become more common in populations.
This process can occur even if populations have only partially developed reproductive isolation during a period of geographic separation. When they come into contact, the production of low-fitness hybrids provides the selective force that drives further divergence. The concept of reinforcement was initially proposed by Alfred Russel Wallace and later significantly developed by Theodosius Dobzhansky.
How Reinforcement Drives Speciation
Reinforcement directly contributes to speciation by strengthening the barriers that prevent gene flow between diverging populations. When individuals from two partially separated groups produce hybrids with reduced viability or fertility, selection favors individuals who avoid these unproductive matings. This selective pressure leads to the evolution of stronger pre-zygotic reproductive barriers.
These barriers can manifest in various ways, such as changes in mating rituals, where courtship displays or songs become distinct enough to prevent interbreeding. Shifts in habitat preferences or differences in breeding times may also evolve, ensuring that individuals from different populations do not encounter each other during their reproductive periods. By reducing the frequency of hybrid formation, reinforcement limits the exchange of genes between the diverging groups.
Avoiding hybrid formation offers an evolutionary advantage. Organisms that invest reproductive resources into offspring with low fitness reduce their overall contribution to the next generation. Therefore, genetic traits that lead to assortative mating, where individuals prefer mates from their own population, are favored by natural selection. This reduction in gene flow, driven by the disadvantage of hybrids, ultimately leads to complete reproductive isolation and distinct species.
Examples of Reinforcement in Nature
Evidence for reinforcement has been observed across various taxa, including vertebrates, invertebrates, plants, and fungi. A well-studied example involves Drosophila fruit flies, where species often show stronger pre-zygotic isolation in areas of range overlap (sympatry) compared to separate areas (allopatry). This pattern, known as reproductive character displacement, suggests selection against hybrids leads to enhanced mate discrimination in sympatric zones. For instance, Drosophila species in overlapping regions exhibit more pronounced differences in courtship behaviors or pheromone signals, making interbreeding less likely.
Another instance is stickleback fish. In freshwater lakes, two forms—benthic (bottom-dwelling) and limnetic (open-water)—have diverged. Where these forms coexist, stronger assortative mating occurs. This is thought to be a result of reinforcement, as hybrids are often less adapted to either specific niche. Studies in Neurospora fungi also provide strong evidence, with sympatric pairs showing lower reproductive success in crosses compared to allopatric pairs, and no hybrids found in nature despite close proximity.